You'll first notice this insect in its pretty butterfly form. Small and white with black wing tips, they land often on choice plants (broccoli, cabbage, Brussels sprouts, and cauliflower are the faves) to deposit yellow, cone-shaped eggs. Once the larvae emerge, usually in five to seven days, they display a voracious appetite and will leave your veggies in tattered rags. They also deposit masses of wet, greenish-brown excrement deep among leaves...not nice.
Very few gardeners can say they've never had these velvety green, chewing pests dining on their cole crops. Entire broccoli plants can be devoured in a day or two in bad cases of infestation. But there are a few ways you can minimize or even alleviate the damage:
At the first sign of the butterflies flitting around your crops, get out the row cover. The lightest weight of cloth is preferable, unless you're experiencing a cold snap. The fabric will provide a barrier against the butterflies laying eggs on your veggies and will therefore halt the entire lurid cycle. (Sticky traps are also effective in catching the butterflies, but you might end up catching beneficial insects as well.)
Alternatives to row covers include releasing beneficial insects such as trichogramma wasps. They will stick around as long as there's something to eat, then move along. Other predatory insects and bug-eating birds will feed on the larvae as well.
Bt (Bacillus thuringiensis) works well but it doesn't begin working immediately upon application, and the liquid form must be reapplied often. Bt shuts down the digestive system of targeted insects, so it takes a few days to kick in.
Spot check your plants frequently on the underside of the leaves; this is where the caterpillar form of this pest likes to hang out and munch out. Handpick them if you're not too disgusted by doing that - and if there aren't too many. If you're using Bt, be sure to spray right onto the offending caterpillars, since many of the aforementioned vegetables tend to have waxy, somewhat water-repellent leaves. Use Bt in the late afternoon or early evening, after the bees have gone to bed.
Some studies have indicated that certain varieties of cabbage show limited resistance to cabbageworms: Mammoth Red Rock, Chieftan Savoy, and Savoy Perfection Drumhead.
Unless controlled, you'll end up with these nasty cabbageworms for pretty much the entire growing season. Keep a continual eye on your plants for any signs of munching. Also, check for worms deep into your cabbage, broccoli and cauliflower heads when harvesting. For the sake of everyone you cook for, soak all cole crops in salt water for an hour before cooking; otherwise you might end up serving an unexpected side dish of worm.
(Dave's Garden member DiOhio took some great photos of the imported cabbageworm - and its destruction - which I've used here.)